1. Field of the Invention
The present invention relates to a color image-forming material and a lithographic printing plate precursor. More specifically, the present invention relates to a color image-forming material and a lithographic printing plate precursor, from which plate-making can be directly made by scanning an infrared laser based on digital signals of a computer or the like and which are usable for printing without passing through a development processing step after exposure.
2. Background Art
The lithographic printing plate in general consists of a lipophilic image area of receiving an ink in the printing process and a hydrophilic non-image area of receiving a fountain solution The lithographic printing is a printing method utilizing the repulsion between water and oily ink from each other, where the lipophilic image area of the lithographic printing plate and the hydrophilic non-image area are formed as an ink-receiving part and a fountain solution-receiving part (ink non-receiving part), respectively, to cause difference in the ink adhesion on the surface of the lithographic printing plate, an ink is attached only to the image area and thereafter, the ink is transferred to a material on which the image is printed, such as paper, thereby performing printing.
For producing this lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (image recording layer) has been heretofore widely used. Usually, a lithographic printing plate is obtained by a plate-making method where the lithographic printing plate precursor is exposed through an original image such as lith film and while leaving the image recording layer in the image area, the image recording layer in the non-image area is dissolved and removed with an alkaline developer or an organic solvent to reveal the hydrophilic support surface.
In the plate-making process using a conventional lithographic printing plate precursor, a step of dissolving and removing the non-image area with a developer or the like according to the image recording layer must be provided after exposure but as one of the problems to be solve, it is demanded to dispense with or simplify such an additional wet processing. Particularly, the treatment of waste solution discharged accompanying the wet processing is recently a great concern to the entire industry in view of consideration for global environment, and the demand for solving the above-described problem is becoming stronger.
As for the non-processing (non-development) type dispensable with the wet processing, a lithographic printing plate precursor having an image recording layer of which affinity for fountain solution or ink changes on the surface upon exposure, and being capable of printing without removing the image recording layer has been proposed.
Also, as one simple and easy plate-making method, a method called on-press development has been proposed, where an image recording layer allowing for removal of the non-image area of a lithographic printing plate precursor during a normal printing process is used and after exposure, the non-image area is removed on a printing press to obtain a lithographic printing plate.
Specifically, the on-press development method includes, for example, a method using a lithographic printing plate precursor having an image recording layer dissolvable or dispersible in a fountain solution, an ink solvent or an emulsified product of fountain solution and ink, a method of mechanically removing the image recording layer by the contact with rollers or a blanket cylinder of a printing press, and a method of weakening the cohesion of the image recording layer or adhesion between the image recording layer and the support by the impregnation of a fountain solution, an ink solvent or the like and then mechanically removing the image recording layer by the contact with rollers or a blanket cylinder.
In the present invention, unless otherwise indicated, the “development processing step” indicates a step where, by using an apparatus (usually an automatic developing machine) except for a printing press, the infrared laser unexposed portion of the lithographic printing plate precursor is removed through contact with a liquid (usually an alkaline developer) to reveal the hydrophilic support surface, and the “on-press development” indicates a method or step where, by using a printing press, the infrared laser unexposed portion of the lithographic printing plate precursor is removed through contact with a liquid (usually a printing ink and/or a fountain solution) to reveal the hydrophilic support surface.
However, when an image recording layer in a conventional image recording system utilizing ultraviolet ray or visible light is used, the image recording layer is not fixed even after exposure and therefore, the lithographic printing plate precursor after exposure must be stored in a completely light-shielded state or constant-temperature condition until loading it on a printing press but this is cumbersome and takes time.
On the other hand, a digitization technique of electronically processing, storing and outputting image information by using a computer has been recently widespread and various new image-output systems coping with such a digitization technique have been put into practical use. Along with this, a computer-to-plate technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser light, and a lithographic printing plate precursor is scan-exposed by this light to directly produce a lithographic printing plate with no intervention of a lith film. Accordingly, one of important technical problems to be solved is to obtain a lithographic printing plate precursor suitable for such a technique.
In this way, the demand for simplification, dry processing or non-processing of the plate-making work is recently ever-stronger in view of both consideration for global environment and adaptation to digitization.
Recently, high output lasers such as semiconductor laser of emitting an infrared ray at a wavelength of 760 to 1,200 nm and YAG laser are inexpensively available, and a method using such a high output laser as the image recording means is a promising method for producing a lithographic printing plate by scan-exposure which is easy to integrate into the digitization technique.
In conventional plate-making methods, a photosensitive lithographic printing plate precursor is imagewise exposed with light at low to medium intensity, as a result, a photochemical reaction occurs in the image recording layer to cause imagewise physical change, whereby an image is recorded.
On the other hand, in the method using the high output laser, a large quantity of light energy is irradiated on the exposure region in a very short time to efficiently convert the light energy into heat energy, as a result, by the effect of heat generated, thermal change such as chemical change, phase change and change in morphology or structure is caused and such change is used for the image recording. Accordingly, although the image information is input by the effect of light energy of laser light or the like, the image recording is performed in the state that not only the light energy but also a reaction by the heat energy are utilized. The recording system using the heat generation by such high power density exposure is usually called “heat-mode recording”, and the conversion of from light energy to heat energy is called “light-to-heat conversion”. In the present invention, such an image recording layer is also called an image recording layer.
The plate-making method using the heat-mode recording is greatly advantageous in that the image recording layer is not photosensitized with light of normal intensity level, such as room lighting, and fixing of the image recorded by high intensity exposure is not essential. In other words, the lithographic printing plate precursor used for heat-mode recording is free from fear that the image recording layer is photosensitized with room light before exposure, and fixing of the image after exposure is not essential. Therefore, for example, when an image recording layer which is insolubilized or solubilized upon exposure by a high output laser is used and the plate-making process of imagewise processing the exposed image recording layer to produce a lithographic printing plate is performed by on-press development, a printing system where even if the image recording layer after exposure is exposed to ambient light in a room, the image is not affected can be established. Accordingly, it is expected that when the heat-mode recording is utilized, a lithographic printing plate precursor suitably used for on-press development can be obtained.
With respect to this technique, for example, Patent Document 1 (Japanese Patent No. 2,938,397) describes a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer obtained by dispersing hydrophobic thermoplastic polymer particles in a hydrophilic binder. In Patent Document 1, it is stated that the lithographic printing plate precursor can be exposed to an infrared laser to cause coalescent of hydrophobic thermoplastic polymer particles by the effect of heat and thereby form an image, loaded on a cylinder of a printing press, and then on-press developed with a fountain solution and/or an ink.
However, the method of forming an image by the coalescence through mere heat fusion of fine particles is found to have a problem that the image strength is weak and the press life is insufficient, despite good on-press developability.
To solve this problem, it is proposed to improve the press life by utilizing a polymerization reaction. For example, Patent Document 2 (JP-A-2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)) describes a lithographic printing plate precursor comprising a hydrophilic support having thereon an image recording layer (heat-sensitive layer) containing polymerizable compound-enclosing microcapsules. Furthermore, Patent Document 3 (JP-A-2002-287334) describes a lithographic printing plate precursor comprising a support having provided thereon an image recording layer (photosensitive layer) containing an infrared absorbent, a radial polymerization initiator and a polymerizable compound.
On the other hand, as for the non-processing type lithographic printing plate precursor, a technique of enhancing the visibility between the exposed area and the unexposed area is demanded and, for example, Patent Document 4 (JP-A-11-277927) describes a non-processing type lithographic printing plate precursor capable of color image recording by the color formation due to acid, base or radical upon infrared laser exposure. However, in the light of restriction from the work environment where the non-processing type lithographic printing plate precursor is used, it is not sufficient that the exposed area merely causes color change with respect to the unexposed area. The technical problem to be solved is to satisfy at least the following conditions: (a) the color change gives lightness difference ΔL≧4, (b) the color change does not occur under white light such as fluorescent light (good white light stability), (c) the color change does not occur during storage of an unexposed plate (good storage stability), and (d) particularly, in application to an on-press development type lithographic printing plate precursor, the color is changed to be colorless or thin-colored at the on-press development and even if only a part is mingled into the printed matter, this causes no evil effect, for example, color change. A technique for color image formation satisfying these various conditions is being demanded.